1. Field of the Invention
The invention relates to transmitters of laser radiation and, more particularly, the invention is concerned with making transmitters of laser radiation eyesafe.
2. Description of the Prior Art
It is a recognized problem that laser beams can damage the eyes and the damage can be caused in a small fraction of a second by a high energy laser beam. Tests on rabbits to determine the harmful effect of lasers have produced severe thermal lesions in the rabbits' eyes. One reason for this, aside from the transmitted intensity of the laser beam, is that pigmented tissue in the eye's retina absorbs more energy from laser beams than does ordinary tissue. A second reason is that spatially coherent radiation focuses to a diffraction limited spot which is very small. This in turn means a very high power density. Thus, the choroid membrane of the eyeball, which contains pigment cells, is very susceptible to laser damage. One suggested means for protecting the eye against laser radiation is goggles having thick opaque lens with a tiny hole in the center of the lens. The laser beam has to go through the center hole to reach the eye. This is unlikely and a cap shield around the center hole lends even more protection. However, it is still possible for a laser beam to enter the small hole and should this happen, the macula, or point of sharp focus on the retina of the eye, would be damaged. The victim would thereafter be able to see light and dark and gross objects, but the ability to focus sharply would be destroyed. It has also been suggested that narrow-band optical fibers could be designed to attentuate the laser beam to safe levels while still passing sufficient other radiation to allow the person looking through the filters to see.
An attempt was made to convert a coherent beam of laser radiation into spatially incoherent radiation by reflecting the beam off a gold surface which had been dimpled by dropping 15 micron diameter steel balls on the surface. The dimpled gold diffusing surface area was tested by imaging a YAG laser on it, at a power density in excess of 100 megawatts per square centimeter (100 MW/CM.sup.2). The surface structure was destroyed by evaporation of the gold.
There have also been protective devices disclosed in the art which function like a shutter. The shutter will supposedly permit full use of an optical system until needed and then the shutter will be capable of closing rapidly in response to harmful radiation. Hopefully, the shutterlike device will operate quickly enough to prevent eye damage. A number of such devices are disclosed in U.S. Pat. No. 3,561,842 issued to Billy M. Horton wherein the optical quality of both reflective and transmissive optical elements are destroyed by high intensity radiation. In the Horton devices, a polished surface material such as silicon monoxide is evaporated from a quartz lens by the radiation, leaving a roughened surface exposed that provides an optically diffusing surface that disrupts the beam from which protection is needed. This approach has the disadvantage, among others, of rendering the optical device using it inoperative. Also, this approach would alter the amplitude, frequency and polarization of the radiation so it would not be employed in a communications or rangefinder system.
Diffuser lens elements are also known in the camera art. These lens, as disclosed in U.S. Pat. No. 4,111,561 issued to William T. Plummer, have a roughened diffuser area that reduces the intensity of the light coming in on the optical axis of the camera. Such inventions are for use in cameras that utilize incoherent radiation and they would have no application in the protection of eyes from laser devices.
There is to date no satisfactory laser apparatus available for eliminating the high power, or energy density, on the retina of the eye. Neither is there a laser transmitter available for providing, at close ranges, a constant power, or energy density, on the retina due to the radiation source being focused to a finite image rather than a diffraction limited spot on the retina of the eye.